1. Field of the Invention
The present invention is concerned with improvements in or relating to extruders and through feed mixers.
2. Prior Art
Through feed mixers, which are adapted to mix plastics and rubbery materials and provide a continuous supply of well mixed and plasticized materials, are becoming more common in the plastics and rubber industries, replacing to some extent the traditional batch mixer, e.g. of the "Banbury" type.
There are two commonly proposed types of through feed mixers at present in use. The first comprises an extrusion type screw, usually of a long L/D ratio, to which material is fed and in which the material is mixed and then extruded to give a desired product. It is possible to design such a screw so that it will perform well on a particular grade of material, but the design is very critical, and small variations in the material will substantially affect the mixing performance of the screw. To give the mixer any degree of versatility, a collection of screws of different characteristics is needed. Additionally, both the specific work put into material passed through the mixer and the output of the mixer are determined by the rate of rotation of the screw.
The second type of mixer, an example of which is disclosed in U.S. Pat. No. 3,154,808, comprises two parallel rotors comprising mixing and feeding sections. In such mixers, which are generally known as continuous mixers, the output is largely dependent on the amount of material fed to the mixer, and is, for a wide range of outputs, almost independent of the speed of the rotors. However, continuous mixers usually work in conjunction with other equipment, such as extruders, and it is in practice extremely difficult to ensure that the output of the mixer is precisely correct for the input of the extruder or the like. French Pat. No. 2,204,446 describes a mixer device utilizing parallel, circular eccentrically rotated, or generally oval-shaped rollers or screws that are tapered from their inlet ends to their outlet ends.
One of the various objects of the present invention is to provide an improved mixer which can readily be arranged to supply mixed plastics or similar material at a desired rate.
Another of the various objects of the present invention is to provide an improved mixer in which the specific work put into mixed plastic or similar material can be readily altered.